[AndyJackson]

Almost certainly sensible rules for what to do in an emergency

Actually it is not sensible. The one thing that is designed for an earthquake is a nuclear reactor. Now you have to recover from an earthquake without any power.

And the one thing required to keep a nuclear plant safe is electric power. Now you are dependent upon the grid.

It is actually dumb, dumb, dumb. But folks don't think things through. Before the Tresher accident the first thing a sub did with flooding was scram the reactor. Then the ship sank because of lack of propulsion. The rules changed.

[matt04]

Hence the redundant onsite emergency diesel generators. The article stated the plant was running on four of them after the reactor was scrammed.

[sefarkas]

Actually it is not sensible.

The power grid is a superior source of reliable electric power in comparison to on-site sources. The power grid has any number of generators as well as switching equipment to maintain power to the loads. A single nuclear plant running at full power will only draw about 50MWe from the power grid, mostly to run pumps. The load is less at shutdown. The power-grids can easily handle a single nuclear site electric power load. Most nuclear plants have two circuits coming into the station and one going out (for each reactor/turbine pair). The three circuits are fairly independent. A nuclear power plant pushing 800MWe to 1200MWe out to the power grid has to be connected to that power grid -- you cannot sit and spin a turbine at full power with no load, or even partial power for that matter. The circuits coming in from off-site are connected to the same AC bus that the on-site DG is connected to, so that if the off-site circuit trips, the DG picks up that load. Two safety-systems in the nuclear plant; thus, two DGs. Either safety-system is sufficient. Safety-systems are meant to be independent to avoid common-cause failures, so cross-connecting is physically possible, but rarely done.